This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A171581 #6 Nov 13 2015 11:08:12
%S A171581 1,2,3,4,6,8,9,10,12,14,15,16,18,20,21,22,24,25,26,27,28,30,32,33,34,
%T A171581 35,36,38,39,40,42,44,45,46,48,49,50,51,52,54,55,56,57,58,60,62,63,64,
%U A171581 65,66,68,69,70,72,74,75,76,77,78,80,81,82,84,85,86,87,88,90,91,92,93,94
%N A171581 The natural numbers without primes > 3.
%C A171581 1, 2, 3 and the composites A002808.
%t A171581 With[{nn=100},Join[{1,2,3},Complement[Range[4,nn],Prime[Range[ PrimePi[ nn]]]]]] (* _Harvey P. Dale_, Nov 18 2013 *)
%o A171581 (PARI) is(n)=if(n<5,n>0,!isprime(n)) \\ _Charles R Greathouse IV_, Nov 13 2015
%K A171581 nonn,easy,less
%O A171581 1,2
%A A171581 _Juri-Stepan Gerasimov_, Dec 12 2009, Dec 15 2009